If several people reply, and they all agree, that's one way to know. But it can be a problem. Sometimes people post very
wrong answers.
I don't like to see people posting when they're clearly guessing, without saying so. It's OK to guess at an answer if you want, but
you should say so. "I'm not sure, but I think..."
If you see an answer and it's clearly wrong, be sure to reply and politely but firmly say so. Be careful in your explanation so the the question-asker understands that you know what you're talking about. Give references if you can.